Energy storage devices having enhanced specific energy and associated methods

ABSTRACT

Ultracapacitor electrodes having an enhanced electrolyte-accessible surface area are provided. Such electrodes can include a porous substrate having a solution side and a collector side, the collector side operable to couple to a current collector and the solution side positioned to interact with an electrolytic solution when in use. The electrode can also include a conductive coating formed on the solution side of the porous substrate. The coating can have a first side positioned to interact with an electrolytic solution when in use and a second side opposite the first side. The coating can have discontinuous regions that allow access of an electrolyte solution to the second side during use to enhance electrolyte-accessible surface area of the conductive coating.

TECHNICAL FIELD

Embodiments described herein relate generally to energy storage devices.

BACKGROUND

Energy storage devices, including batteries and capacitors, are oftenused in various electronic devices. Capacitors, for example, are widelyused for applications ranging from electrical circuitry and powerdelivery to voltage regulation and battery replacement. As capacitortechnology has continued to develop, several types have emerged. As oneexample, electric double-layer capacitors (EDLCs), also referred to asultracapacitors, are characterized by high energy storage and powerdensity, small size, and low weight and have thus become promisingcandidates for use in several applications.

Ultracapacitors and similar high-surface-area charge storage devices canbe used for a variety of purposes, including in microelectronics tostore energy, for electrical bypassing in electric circuits, as part ofcircuitry for power delivery, as a memory storage element, and for ahost of other functions. One advantage of ultracapacitors over batteriesis that ultracapacitors can be charged and discharged quickly becausethey do not rely on chemical reactions to store energy, and they don'tdegrade significantly over their lifetime; even when charged anddischarged rapidly. Ultracapacitors are also less sensitive totemperature than are batteries.

Although current ultracapacitors have very high specific power, theyhave a relatively low specific energy (Wh/kg) compared to batterysystems and thus have limited applications for energy storage devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a method of making an ultracapacitorelectrode having an enhanced electrolyte-accessible electricallyconductive surface area in accordance with one invention embodiment;

FIG. 2 is a schematic view of a section of an ultracapacitor electrodein accordance with another invention embodiment;

FIG. 3 is a schematic view of an ultracapacitor electrode in accordancewith another invention embodiment;

FIG. 4A is a schematic view of an ultracapacitor electrode being made inaccordance with yet another invention embodiment;

FIG. 4B is a schematic view of an ultracapacitor electrode being made inaccordance with yet another invention embodiment;

FIG. 4C is a schematic view of an ultracapacitor electrode being made inaccordance with yet another invention embodiment;

FIG. 5 is a schematic view of an ultracapacitor electrode in accordancewith another invention embodiment;

FIG. 6 is a schematic view of an ultracapacitor device in accordancewith another invention embodiment; and

FIG. 7 is a schematic view of a capacitor in accordance with anotherinvention embodiment.

DESCRIPTION OF EMBODIMENTS

Although the following detailed description contains many specifics forthe purpose of illustration, a person of ordinary skill in the art willappreciate that many variations and alterations to the following detailscan be made and are considered to be included herein.

Accordingly, the following embodiments are set forth without any loss ofgenerality to, and without imposing limitations upon, any claims setforth. It is also to be understood that the terminology used herein isfor the purpose of describing particular embodiments only, and is notintended to be limiting. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as commonlyunderstood by one of ordinary skill in the art to which this disclosurebelongs.

As used in this specification and the appended claims, the singularforms “a,” “an” and “the” include plural referents unless the contextclearly dictates otherwise. Thus, for example, reference to “a layer”includes a plurality of such layers.

In this disclosure, “comprises,” “comprising,” “containing” and “having”and the like can have the meaning ascribed to them in U.S. Patent lawand can mean “includes,” “including,” and the like, and are generallyinterpreted to be open ended terms. The terms “consisting of” or“consists of” are closed terms, and include only the components,structures, steps, or the like specifically listed in conjunction withsuch terms, as well as that which is in accordance with U.S. Patent law.“Consisting essentially of” or “consists essentially of” have themeaning generally ascribed to them by U.S. Patent law. In particular,such terms are generally closed terms, with the exception of allowinginclusion of additional items, materials, components, steps, orelements, that do not materially affect the basic and novelcharacteristics or function of the item(s) used in connection therewith.For example, trace elements present in a composition, but not affectingthe compositions nature or characteristics would be permissible ifpresent under the “consisting essentially of” language, even though notexpressly recited in a list of items following such terminology. Whenusing an open ended term, like “comprising” or “including,” it isunderstood that direct support should be afforded also to “consistingessentially of” language as well as “consisting of” language as ifstated explicitly.

“The terms “first,” “second,” “third,” “fourth,” and the like in thedescription and in the claims, if any, are used for distinguishingbetween similar elements and not necessarily for describing a particularsequential or chronological order. It is to be understood that the termsso used are interchangeable under appropriate circumstances such thatthe embodiments described herein are, for example, capable of operationin sequences other than those illustrated or otherwise described herein.Similarly, if a method is described herein as comprising a series ofsteps, the order of such steps as presented herein is not necessarilythe only order in which such steps may be performed, and certain of thestated steps may possibly be omitted and/or certain other steps notdescribed herein may possibly be added to the method.

The terms “left,” “right,” “front,” “back,” “top,” “bottom,” “over,”“under,” and the like in the description and in the claims, if any, areused for descriptive purposes and not necessarily for describingpermanent relative positions. It is to be understood that the terms soused are interchangeable under appropriate circumstances such that theembodiments described herein are, for example, capable of operation inother orientations than those illustrated or otherwise described herein.The term “coupled,” as used herein, is defined as directly or indirectlyconnected in an electrical or nonelectrical manner. Objects describedherein as being “adjacent to” each other may be in physical contact witheach other, in close proximity to each other, or in the same generalregion or area as each other, as appropriate for the context in whichthe phrase is used. Occurrences of the phrase “in one embodiment,” or“in one aspect,” herein do not necessarily all refer to the sameembodiment or aspect.

As used herein, the term “substantially” refers to the complete ornearly complete extent or degree of an action, characteristic, property,state, structure, item, or result. For example, an object that is“substantially” enclosed would mean that the object is either completelyenclosed or nearly completely enclosed. The exact allowable degree ofdeviation from absolute completeness may in some cases depend on thespecific context. However, generally speaking the nearness of completionwill be so as to have the same overall result as if absolute and totalcompletion were obtained. The use of “substantially” is equallyapplicable when used in a negative connotation to refer to the completeor near complete lack of an action, characteristic, property, state,structure, item, or result. For example, a composition that is“substantially free of” particles would either completely lackparticles, or so nearly completely lack particles that the effect wouldbe the same as if it completely lacked particles. In other words, acomposition that is “substantially free of” an ingredient or element maystill actually contain such item as long as there is no measurableeffect thereof.

As used herein, the term “about” is used to provide flexibility to anumerical range endpoint by providing that a given value may be “alittle above” or “a little below” the endpoint.

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary.

Concentrations, amounts, and other numerical data may be expressed orpresented herein in a range format. It is to be understood that such arange format is used merely for convenience and brevity and thus shouldbe interpreted flexibly to include not only the numerical valuesexplicitly recited as the limits of the range, but also to include allthe individual numerical values or sub-ranges encompassed within thatrange as if each numerical value and sub-range is explicitly recited. Asan illustration, a numerical range of “about 1 to about 5” should beinterpreted to include not only the explicitly recited values of about 1to about 5, but also include individual values and sub-ranges within theindicated range. Thus, included in this numerical range are individualvalues such as 2, 3, and 4 and sub-ranges such as from 1-3, from 2-4,and from 3-5, etc., as well as 1, 2, 3, 4, and 5, individually.

This same principle applies to ranges reciting only one numerical valueas a minimum or a maximum. Furthermore, such an interpretation shouldapply regardless of the breadth of the range or the characteristicsbeing described.

Reference throughout this specification to “an example” means that aparticular feature, structure, or characteristic described in connectionwith the example is included in at least one embodiment. Thus,appearances of the phrases “in an example” in various places throughoutthis specification are not necessarily all referring to the sameembodiment.

Example Embodiments

An initial overview of technology embodiments is provided below andspecific technology embodiments are then described in further detail.This initial summary is intended to aid readers in understanding thetechnology more quickly but is not intended to identify key or essentialfeatures of the technology, nor is it intended to limit the scope of theclaimed subject matter.

As has been described, current ultracapacitors have very high specificpower but relatively low specific energy (Wh/kg) compared to batterysystems and thus have limited applications for energy storage devices.Current commercially available ultracapacitors are typically carbonbased, although silicon based ultracapacitors have recently beencontemplated. Silicon based ultracapacitors typically employ a poroussilicon substrate material coated with a second material that isconductive and inert. Since carbon is naturally conductive and highlyinert, no passivation is required for carbon based electrodes and noadditional layers are used. The coating of second material in anactivated electrode of a silicon based ultracapacitor typically has onlyone surface that is active and the electrolyte forms a double layer onthat surface. The other surface is inactive since it is in contact withthe porous silicon.

Aspects of the present disclosure provide methods and techniques forincreasing the specific energy of an ultracapacitor, such as, forexample, increasing the capacitance, decreasing the device weight, andthe like. In one embodiment, specific energy can be increased byincreasing the available surface area of the ultracapacitor electrodes.In another embodiment, the weight of the device can be decreased byremoving at least a portion of the electrode material to provide regionsthat can be filled with electrolyte. Since the electrolyte has a lowermass density than the electrode, a packaged device utilizing one or moreof these approaches can have a higher specific capacitance and/or energyas compared to ultracapacitor electrodes that have not been so treated.

In one embodiment, as is shown in FIG. 1 for example, a method of makingan ultracapacitor electrode having an enhanced electrolyte-accessibleelectrically conductive surface area can include 102 depositing aconductive coating onto a porous substrate, 104 forming at least oneopening in the conductive coating, and 106 selectively etching theporous substrate through the at least one opening to provide electrolyteaccessibility to a side of the conductive coating substantially orientedtoward the porous substrate. Thus by etching the porous substratethrough openings in the conductive coating, regions of the conductivecoating that were previously contacting, or substantially contacting,the porous substrate can be made accessible to electrolyte therebyincreasing the available surface area of the conductive coating.Additionally, the portions of the porous substrate that have been etchedaway create open regions that can accommodate a greater amount ofelectrolyte. These effects taken separately or together can increase thespecific energy of the ultracapacitor.

The selective etch can thus remove at least a portion of the poroussubstrate to allow the backside or previously inaccessible surface ofthe conductive material to be active and allow the use of this innersurface as additional surface area for the double layer to form. Thus,the conductive coating combined with the porous substrate allows aselective etching to form increased surface area to an extent that wouldnot be possible using a single material. Many traditional processesusing a porous electrode material are limited to a surface area that wasessentially fixed at the creation of the porous material itself. Surfacearea cannot generally be significantly enhanced beyond such initialprocessing.

It is noted that in some embodiments only a portion of the poroussubstrate is selectively etched away leaving a combination of poroussubstrate and conductive material having a high surface area. In otherembodiments substantially all or all of the porous substrate isselectively etched away leaving at least primarily a conductive materialhaving a high surface area. It is also noted that the choice ofmaterials used in constructing the porous substrate and the conductivecoating can be dependent upon the selective etching process utilized, oralternatively, that the selective etching process can be dependent uponthe materials used for the conductive coating and the porous substrate.Thus in one embodiment, the etching process can selectively etch theporous substrate while leaving the conductive coating at leastsubstantially undisturbed.

As is shown in FIG. 2, a conductive coating 202 having a first surface212 is formed on a substrate 204. The first surface 212 is generallyoriented so as to face in the same direction as a surface of thesubstrate 204 underlying the conductive coating 202, and upon which itwas deposited. Typically, the first surface 212 is not, and has notbeen, in contact with any other material prior to introduction of anelectrolyte into the device. The substrate can be a porous substrateupon which the conductive coating is formed, or the substrate can be anon-porous substrate that is made porous following the deposition of theconductive coating. At least one opening 206 can be formed in theconductive coating 202. In one embodiment, the conductive coating can bedeposited as a continuous, or at least substantially continuous, coatingand the at least one opening is subsequently formed therein. In anotherembodiment, the conductive coating can be formed as a discontinuouscoating and openings are thus discontinuities in the coating as it wasformed. Thus openings can be actively formed in the coating, or openingscan be present in the coating as it forms (i.e. passively formed), orboth.

The substrate 204 can be selectively etched through the at least oneopening 206, thus forming one or more voids 208 in the substrate 204.Such voids provide electrolyte accessibility to a side 210 of theconductive coating 202 that was previously in contact with the poroussubstrate 204, or that is substantially oriented toward the poroussubstrate 204 (and therefore facing in an opposite direction, orsubstantially opposite direction than the first surface 212). This side210 of the conductive coating 202 can also be referred to as the secondside. By removal of the substrate material 202 and exposure of thesecond side 210 of the conductive coating 202, the weight of the poroussubstrate 204 is decreased, the electrolyte-accessible surface area ofthe conductive coating 202 is increased, and the electrolyte capacity ofthe electrode is increased.

In another embodiment, an ultracapacitor electrode having an enhancedelectrolyte-accessible surface area is shown in FIG. 3. The electrodecan include a porous substrate 302 having a solution side 304 and acollector side 306. The solution side includes a side of the poroussubstrate that is oriented toward or positioned to interact with anelectrolytic solution when in use. It is contemplated that suchorientation/interaction can include situations where the poroussubstrate directly contacts the electrolytic solution, as well assituations where the porous substrate does not directly contact theelectrolytic solution, as may be the case for an intermediate layer ofmaterial disposed therebetween. The collector side 306 includes a sideof the porous substrate 302 that is operable to couple to a currentcollector, either directly or indirectly. Thus for the specificembodiment shown in FIG. 3, the collector side 306 couples to a currentcollector (not shown) while the three shown solution sides are orientedtoward the electrolytic solution.

A conductive coating 308 is formed on the solution side 304 of theporous substrate 302. The conductive coating 308 has a first side 310and a second side 312. The first side 310 is positioned to interact withan electrolytic solution when in use and the second side 312 is oppositethe first side 310. Note that in the absence of selective etching, thesecond side 312 is in contact with the substrate 302 and is, for themost part, inaccessible to the electrolytic solution. The conductivecoating 308 has discontinuous regions or openings 314 that allow accessof the electrolytic solution to the second side 312 during use toenhance electrolyte-accessible surface area of the conductive coating308. Voids 316 generated by selective etching in the substrate 302 canenhance electrolytic access to the second side 312. It is noted that theconfiguration shown here is merely exemplary and that any electrodedesign where the second side of the conductive coating is electrolyteaccessible is considered to be within the present scope.

One non-limiting example of an alternative design for an electrode isshown in FIGS. 4A-C. FIG. 4A shows a substrate 402 having a conductivecoating 404 formed thereon. A portion of the conductive coating 404 hasbeen removed to uncover an exposed portion 406 of the substrate 402, asis shown in FIG. 4B. The substrate 402 is then selectively etchedstarting at the exposed portion 406. Such selective etching removes atleast a portion of the substrate 402 to create an electrolyte-accessiblesurface 408 of the conductive coating 404 where the substrate 402 hasbeen etched away (See FIG. 4C). These portions of the conductive coating404 can be free-standing or self-supporting as the underlying substrate402 has been removed. It is understood that any amount of the substratecan be removed, depending upon the desired design of the electrode.Additionally, substantial portions of the substrate can be removedthrough a plurality of openings in the conductive coating to form afree-standing or self-supporting structure.

In some embodiments, following the removal of substrate material usingthe secondary activation step, a subsequent operation to depositadditional conductive coating on the newly exposed surface area of thesubstrate material can be made. In such event, the depositing of theconductive coating can be made (i.e. repeated) through the openings inthe conductive coating and onto newly exposed portions of the poroussubstrate to further increase specific energy. For example, a secondaryconductive coating can be formed on etched portions of the poroussubstrate through the openings in the conductive coating. Secondaryopenings can then be formed in the secondary conductive coating throughwhich a selective etch can be used to further etch the porous substrate.In this way, accessibility to the side of the secondary conductivecoating oriented toward the etched porous substrate can be provided toincrease surface area, and regions opened up through the etching of theporous substrate can provide additional electrolyte space. In someembodiments these processes can be repeated until all, or substantiallyall, of the porous substrate has been replaced by a subsequentconductive coating.

As is shown in FIG. 5, for example, an electrode can have a substrate502 having a conductive coating 504 formed thereon. The conductivecoating 504 has a first side 506 and a second side 508. The first side506 is positioned to interact with an electrolytic solution when in useand the second side 508 is opposite the first side 506. The conductivecoating 504 has discontinuous regions or openings 510 that allow accessof the electrolytic solution to the second side 508 during use toenhance electrolyte-accessible surface area of the conductive coating504. Voids 512 created by selective etching in the substrate 502 canenhance electrolytic access to the second side 508. To further enhancethe electrolyte-accessible surface area, a secondary conductive coating514 can be deposited on surfaces of the voids 512 (i.e. etched surfacesof the substrate 502) through the openings 510. Secondary openings 516(or in some cases, at least one secondary opening) are created in thesecondary conductive coating 514, either via an etching process or bydepositing the secondary conductive coating as a discontinuous layer.Further portions of the substrate 502 can then be selectively etchedthrough the secondary openings 516 to generate secondary voids 518, thusproviding electrolyte accessibility to a side of the secondaryconductive coating 514 substantially oriented toward the etchedsubstrate that was previously inaccessible. These processes can befurther repeated to generate progressively more electrolyte-accessiblesurface area with each iteration.

It is also noted that, for materials that do not interact with theelectrolyte solution in a beneficial way (e.g. non-doped poroussilicon), the inside surfaces of voids (e.g. 512 in FIG. 5) where thesubstrate material will be exposed to electrolyte may not provide usefulsurface area. In some embodiments, therefore, conductive coating can bedeposited onto the inside surfaces of such voids (with or withoutfurther selective etch processing) to increase the usable surface areacapable of interacting with the electrolyte.

A variety of substrate materials are contemplated, and any materialcapable of supporting a conductive coating and allowing selectiveetching to occur in a manner suitable for the creation of an electrodeis considered to be within the present scope. In one embodiment, thesubstrate can be formed as a porous substrate. Non-limiting examples ofsuch porous substrates can include carbon nanotubes, carbon nanofibers,graphene structures, particulate materials, and the like. In the case ofparticulate materials, it can be beneficial to hold the particlestogether with a conductive binder. In another embodiment, the substratecan be a nonporous material that is transformed into a porous material.Non-limiting examples of such materials can include nano- ormicro-structured materials, porous etched materials, laser ablatedmaterials, anodized materials, and the like. Suitable starting materialscan include metals, semiconductors, silicon, carbon, silicon carbide,activated carbon, aluminum, and the like, including combinationsthereof. In one specific embodiment, the substrate can be poroussilicon. In another specific embodiment, the porous silicon is doped.Following the creation of the porous substrate, the conductive coatingcan be deposited thereon.

It is also contemplated that in some embodiments the conductive coatingcan be applied to a nonporous substrate followed by selective etching toform an underlying porous substrate. In other embodiments, the nonporoussubstrate can be substantially etched away to form a self-supporting orfree-standing conductive coating electrode, although it is noted thatself-supporting structures can similarly be formed via the etching of anunderlying porous substrate.

A variety of materials suitable for use as conductive coatings arecontemplated, and any material capable of use as an electrode isconsidered to be within the present scope. Non-limiting examples ofconductive coating materials can include metals, metal nitrides, metalsilicides, metal carbides, metal borides, carbon and the like, includingcombinations thereof. Other more specific non-limiting examples caninclude titanium nitride, tungsten nitride, tantalum nitride, and thelike, including combinations thereof. Additionally, the conductivecoating can be applied to any thickness, depending on the desired designand functionality of the resulting electrode. In one embodiment,however, the conductive coating can have an average thickness of fromabout 5 nm to about 50 nm thick. In some embodiments the film may have athickness that is sufficiently thick to allow the conduction ofelectrons (i.e. the conductivity of a thin film is directly proportionalto thickness) and be thin enough so that it does not substantially fillthe porous substrate. In another embodiment the thickness may be lessthan 5 nm thick providing a discontinuous film with naturally occurringopenings for a selective etch. In this embodiment the electrons areconducted by the thin film and at least partially by the poroussubstrate.

While in many cases the conductive coating is deposited onto thesubstrate, in some embodiments the conductive coating can be formed onthe substrate through the interaction of the substrate with an overlyingreactive material. For example, a reactive material can be depositedonto the substrate such that reaction between the substrate and thereactive material forms the conductive coating (i.e. a conductivealloy). In some embodiments reaction can be facilitated by heating thereactive material and the substrate to form the conductive coating at aninterface between the reactive material and the porous substrate. Inthis case the heating can cause intermixing of the two materials to forma beneficial alloy. As one example, porous silicon can be coated withnickel and heated to form a NiSi_(x) material at the Ni—Si interface.Following generation of the conductive coating, the reactive materialcan be at least partially removed to expose the conductive coating. Inthe Si/Ni reaction, unreacted nickel can be removed using an appropriateselective etch, such as H₂SO₄/H₂O₂/H₂O with, for example, a 4:1:60ratio. In another example, the reactive material may be carried in a gasor vapor phase, such as acetylene (C₂H₂). At high temperature theacetylene can react with the porous substrate to form SiC on thesurface. The remaining acetylene can be removed via pumping thesubstrate down to a low pressure.

It can be beneficial to select both the substrate and conductive coatingmaterials with a selective etch process in mind. To generate theincreased surface area on the backside of the conductive coating (i.e.the side contacting or facing the substrate), it is useful to utilize anetch process that etches the substrate while leaving the conductivecoating at least substantially unaffected. If an etch is used thatetches both the substrate and the conductive coating to a substantialextent, the conductive coating can be etched away or chemicallypolished, thus reducing if not eliminating the surface area gainsdescribed herein. As such, in general any etch can be used providedthere is sufficient etch selectivity between the substrate and theconductive coating. In one embodiment, for example, the selective etchcan be a dry etch. In another embodiment, the selective etch can be awet etch.

In one specific example, the substrate material can be porous silicon,the conductive coating can be titanium nitride (TiN), and the etchprocess can be a selective etch for these materials such as XeF₂ for dryetching, KOH for wet etching, and the like. XeF₂, for example, has ahigh selectivity between TiN and porous silicon at low temperature, andcan thus selectively etch the porous silicon without substantiallyaffecting the TiN. In this way, surfaces of the TiN coating facing theporous silicon substrate are now electrolyte-accessible, thus increasingthe surface area of the electrode. It is noted that openings in the TiNcoating can be present due to discontinuous deposition, by an etchprocess that is selective for TiN, or by other techniques, such asmechanical perturbation or thermal cycling.

In another embodiment, a conductive coating can be formed on a substrateby a technique such as atomic layer deposition (ALD). In ALD, the“seeding” layer determines the conformity of the thin film growthprocess. In a typical Volmer-Weber thin-film growth mechanismdiscontinuous islands form up until a critical thickness is reached. Assuch, a conductive coating can be deposited as a discontinuous layerhaving openings that expose the underlying substrate material. As such,the substrate material can be selectively etched without the need forcreating openings in the conductive coating as those openings arepresent upon deposition.

In another embodiment, a substrate material such as porous silicon canbe selectively etched by a process such as stain etching, which willslowly etch holes into the silicon structure where not protected by theconductive coating. Stain etching can be accomplished in a variety ofways, one example of which is a combination of hydrofluoric acid, nitricacid, and water.

As another example, carbon materials such as, for example, graphene,activated carbon, carbon nanotubes (both multi-walled andsingle-walled), carbon nanofibers, other carbon structure with nano ormicro-scale features, and the like, can be assembled into a high surfacearea substrate structure, and then coated with a passivation coatingusing a process such as ALD. Since carbon is generally not reactive withALD precursors, the passivation material coating on carbon surfacesoften form non-continuous films. In order to etch pits into the carbonmaterial, a reactive species (such as oxygen plasma, ozone, etc.) can beutilized. In the areas where the passivation coating is deposited, thereactive species will not etch the carbon. However, in the regions wherethe carbon is exposed, pores and pits will be etched into the carbon atan extent/rate that is governed by the exposure time to the reactivespecies. After this step, in some cases the device can be utilized as is(in the case where the passivation coating is a conductive coating), orin other cases the thin film ALD coating can be dissolved or etched awayto leave a pristine carbon material with a greater surface area andlower weight than when it was formed, which favors better performancefor an electrochemical energy storage device.

Furthermore, the present selectively etched electrode materials can beutilized in a variety of devices, including ultracapacitor electrodes,ultracapacitors, hybrid capacitors, pseudocapacitors, and the like. Inone embodiment, for example, an ultracapacitor having increased specificenergy is provided, as is shown in FIG. 6. The ultracapacitor includesat least two current collectors 602 each coupled to an electrode 604 ashas been described. The current collectors 602 are positioned such thatthe electrodes 604 face one another. An electrolyte solution 606 isdisposed between the electrodes 604 such that the electrolyte contactsat least a portion of the first side 608 and at least a portion of thesecond side 610 of the conductive coatings of each electrode 604. In onespecific embodiment, the electrolyte contacts at least about 5% of thesecond side 610 of the conductive coatings of each electrode 604. Insome embodiments, a separator material can be disposed between the twoelectrodes 604. It is also contemplated that, in some embodiments, aplurality of ultracapacitor cells can be stacked together to create acharged storage system.

In another embodiment, a capacitor having increased specific energy isprovided, as is shown in FIG. 7. Such a capacitor can include a currentcollector 702 coupled to an ultracapacitor electrode 704 as has beendescribed. A faradaic electrode 706 is positioned to face the conductivecoating 708 of the ultracapacitor electrode 704. An electrolyte solution710 is disposed between the faradaic electrode 706 and theultracapacitor electrode 704 such that the electrolyte solution 710contacts at least a portion of the first side 712 and at least a portionof the second side 714 of the conductive coating 708 of theultracapacitor electrode 704 and a surface of the faradaic electrode706. In some embodiments it can be beneficial for the capacities and/orcapacitances of the faradaic electrode and the ultracapacitor electrodeto at least substantially balance.

The faradaic electrode can include any useful matter that isfaradaically active. In one embodiment, for example, the faradaicelectrode can be a pseudocapacitive material such as transition metaloxides, transition metal nitrides, and conducting polymers. Furthernon-limiting examples include nickel oxide, manganese oxide, rutheniumoxide, titanium oxide, niobium oxide, vanadium nitride, polyaniline andthe like. In other embodiments, the faradaically active material may bea battery anode or cathode materials, such as, graphite, lithiatedtransition metal oxides (LiCoO_(x), LiNi_(x)Mn_(y)Co_(z)O, lithiumtitanate, etc.), silicon, tin, phosphates, flurophosphates, silicates,borates, and the like.

A variety of specific methods may be employed in the creation ofelectrodes and energy storage devices, such as capacitors orultracapacitors utilizing such electrodes as described herein. In oneexample, a method of making an ultracapacitor electrode having anenhanced electrolyte-accessible electrically conductive surface area mayinclude depositing a conductive coating onto a substrate, forming atleast one opening in the conductive coating, and selectively etching thesubstrate through the at least one opening to provide electrolyteaccessibility to a side of the conductive coating substantially orientedtoward the porous substrate.

In one example, the substrate can be a porous material at the time thatthe conductive coating is deposited thereon.

In one example, the method can further include depositing a secondaryconductive coating on the etched substrate, forming at least onesecondary opening in the secondary conductive coating, and selectivelyetching the etched substrate through the at least one secondary openingto provide electrolyte accessibility to a side of the secondaryconductive coating substantially oriented toward the etched substrate.

In one example, the method can include repeatedly depositing asubsequent conductive coating on the etched substrate, forming at leastone subsequent opening in the subsequent conductive coating, andselectively etching the etched substrate through the at least onesubsequent opening until substantially all of the substrate is replacedwith the subsequent conductive coating.

In one example, selectively etching the substrate through the at leastone opening further includes removing a sufficient portion of thesubstrate such that at least a portion of the conductive coating isfree-standing.

In one example, the substrate includes a member selected from the groupconsisting of: a semiconductor, a metal, carbon, graphene, activatedcarbon, carbon nanotubes, carbon nanofibers, or combinations thereof.

In one example, the substrate is silicon.

In one example, the method can include doping the silicon.

In one example, depositing the conductive coating further includesdepositing a material selected from the group consisting of: metals,metal nitrides, metal silicides, metal carbides, metal borides, orcombinations thereof.

In one example, depositing the conductive coating further includesdepositing a material selected from the group consisting of: titaniumnitride, tungsten nitride, tantalum nitride, or combinations thereof.

In one example, the selective etch is a dry etch.

In one example, the selective etch is a wet etch.

In one example, the substrate is silicon, the conductive coating istitanium nitride, and the etch is XeF₂ or KOH.

In one example, forming the at least one opening in the conductivecoating further includes depositing the conductive coating onto theporous substrate as a discontinuous coating.

In one example, depositing the conductive coating on the substrateincludes coating a reactive material onto the substrate and reacting thereactive material with the substrate to form a conductive alloy.

In one example, reacting the reactive material with the substratefurther includes heating the reactive material and the substrate to formthe conductive coating at an interface between the reactive material andthe substrate, and removing the reactive material to expose theconductive coating.

In an example embodiment, an ultracapacitor electrode having an enhancedelectrolyte-accessible surface area, can include, a porous substratehaving a solution side and a collector side, the collector side operableto couple to a current collector and the solution side positioned tointeract with an electrolytic solution when in use. The electrode canfurther include a conductive coating formed on the solution side of theporous substrate and having a first side positioned to interact with anelectrolytic solution when in use and a second side opposite the firstside. The conductive coating can have discontinuous regions that allowaccess of an electrolyte solution to the second side during use toenhance electrolyte-accessible surface area of the conductive coating.

In one example, at least a portion of the porous substrate is removed tofurther enhance the electrolyte-accessible surface area of theconductive coating.

In one example, embodiment at least a portion of the conductive coatingis free-standing.

In one example, the electrode may include a secondary conductive coatingformed on an exposed portion of the porous substrate and having asecondary first side positioned to interact with an electrolyticsolution when in use and a secondary second side opposite the secondaryfirst side. The secondary conductive coating can have secondarydiscontinuous regions that allow access of an electrolyte solution tothe secondary second side during use to enhance electrolyte-accessiblesurface area of the secondary conductive coating.

In one example, the porous substrate includes a member selected from thegroup consisting of: a semiconductor, a metal, carbon, graphene,activated carbon, carbon nanotubes, carbon nanofibers, anodizedaluminum, or combinations thereof.

In one example, the porous substrate is porous silicon.

In one example, the porous silicon is doped.

In one example, the porous substrate is a particulate.

In one example, the particulate is held together by the conductivecoating.

In one example, the conductive coating includes a material selected fromthe group consisting of: metals, metal nitrides, metal silicides, metalcarbides, metal borides, carbon or combinations thereof.

In one example, the conductive coating includes a material selected fromthe group consisting of: titanium nitride, tungsten nitride, tantalumnitride, or combinations thereof.

In an example embodiment, an ultracapacitor may include at least twocurrent collectors each coupled to an electrode as recited herein,wherein the current collectors are positioned such that the electrodesface one another, and an electrolyte disposed between the electrodessuch that the electrolyte contacts at least a portion of the first sideand at least a portion of the second side of the conductive coatings ofeach electrode.

In one example, the electrolyte contacts at least about 5% of the secondside of the conductive coatings of each electrode.

In one example embodiment a charge, or energy, storage system caninclude a stack of ultracapacitor cells, including at least oneultracapacitor as recited herein.

In one example embodiment a capacitor can include a current collectorcoupled to an ultracapacitor electrode as recited herein, a faradaicelectrode positioned to face the conductive coating of theultracapacitor electrode, and an electrolyte disposed between thefaradaic electrode and the ultracapacitor electrode such that theelectrolyte contacts at least a portion of the first side and at least aportion of the second side of the conductive coating of theultracapacitor electrode and a surface of the faradaic electrode.

In one example, the faradaic electrode includes at least one of thefollowing: psuedocapacitve, battery anode, or battery cathode material.

In one example, the faradaic electrode includes a material selected fromthe group consisting of: transition metal oxides, transition metalnitrides, conducting polymers, nickel oxide, manganese oxide, rutheniumoxide, titanium oxide, niobium oxide, vanadium nitride, polyaniline,graphite, lithiated transition metal oxides (LiCoO_(x),LiNi_(x)Mn_(y)Co_(z)O, lithium titanate, etc.), silicon, tin,phosphates, flurophosphates, silicates, borates, or combinationsthereof.

While the forgoing examples are illustrative of the specific embodimentsin one or more particular applications, it will be apparent to those ofordinary skill in the art that numerous modifications in form, usage anddetails of implementation can be made without departing from theprinciples and concepts articulated herein. Accordingly, no limitationis intended except as by the claims set forth below.

What is claimed is:
 1. A method of making an ultracapacitor electrodehaving an enhanced electrolyte-accessible electrically conductivesurface area, comprising: depositing a conductive coating onto asubstrate; forming at least one opening in the conductive coating; andselectively etching the substrate through the at least one opening toprovide electrolyte accessibility to a side of the conductive coatingsubstantially oriented toward the porous substrate.
 2. The method ofclaim 1, wherein the substrate is a porous material at the time that theconductive coating is deposited thereon.
 3. The method of claim 1,further comprising: depositing a secondary conductive coating on theetched substrate; forming at least one secondary opening in thesecondary conductive coating; and selectively etching the etchedsubstrate through the at least one secondary opening to provideelectrolyte accessibility to a side of the secondary conductive coatingsubstantially oriented toward the etched substrate.
 4. The method ofclaim 3, further comprising repeatedly: depositing a subsequentconductive coating on the etched substrate; forming at least onesubsequent opening in the subsequent conductive coating; and selectivelyetching the etched substrate through the at least one subsequent openinguntil substantially all of the substrate is replaced with the subsequentconductive coating.
 5. The method of claim 1, wherein selectivelyetching the substrate through the at least one opening further comprisesremoving a sufficient portion of the substrate such that at least aportion of the conductive coating is free-standing.
 6. The method ofclaim 1, wherein the substrate includes a member selected from the groupconsisting of: a semiconductor, a metal, carbon, graphene, activatedcarbon, carbon nanotubes, carbon nanofibers, or combinations thereof. 7.The method of claim 1, wherein the substrate is silicon.
 8. The methodof claim 7, further comprising doping the silicon.
 9. The method ofclaim 1, wherein depositing the conductive coating further comprisesdepositing a material selected from the group consisting of: metals,metal nitrides, metal silicides, metal carbides, metal borides, orcombinations thereof.
 10. The method of claim 1, wherein depositing theconductive coating further comprises depositing a material selected fromthe group consisting of: titanium nitride, tungsten nitride, tantalumnitride, or combinations thereof.
 11. The method of claim 1, wherein theselective etch is a dry etch.
 12. The method of claim 1, wherein theselective etch is a wet etch.
 13. The method of claim 1, wherein thesubstrate is silicon, the conductive coating is titanium nitride, andthe etch is XeF₂ or KOH.
 14. The method of claim 1, wherein forming theat least one opening in the conductive coating further comprisesdepositing the conductive coating onto the porous substrate as adiscontinuous coating.
 15. The method of claim 1, wherein depositing theconductive coating on the substrate comprises: coating a reactivematerial onto the substrate; and reacting the reactive material with thesubstrate to form a conductive alloy.
 16. The method of claim 15,wherein reacting the reactive material with the substrate furthercomprises: heating the reactive material and the substrate to form theconductive coating at an interface between the reactive material and thesubstrate; and removing the reactive material to expose the conductivecoating.
 17. An ultracapacitor electrode having an enhancedelectrolyte-accessible surface area, comprising: a porous substratehaving a solution side and a collector side, the collector side operableto couple to a current collector and the solution side positioned tointeract with an electrolytic solution when in use; and a conductivecoating formed on the solution side of the porous substrate and having afirst side positioned to interact with an electrolytic solution when inuse and a second side opposite the first side, the conductive coatinghaving discontinuous regions that allow access of an electrolytesolution to the second side during use to enhance electrolyte-accessiblesurface area of the conductive coating.
 18. The electrode of claim 17,wherein at least a portion of the porous substrate is removed to furtherenhance the electrolyte-accessible surface area of the conductivecoating.
 19. The electrode of claim 18, wherein at least a portion ofthe conductive coating is free-standing.
 20. The electrode of claim 17,further comprising a secondary conductive coating formed on an exposedportion of the porous substrate and having a secondary first sidepositioned to interact with an electrolytic solution when in use and asecondary second side opposite the secondary first side, the secondaryconductive coating having secondary discontinuous regions that allowaccess of an electrolyte solution to the secondary second side duringuse to enhance electrolyte-accessible surface area of the secondaryconductive coating.
 21. The electrode of claim 17, wherein the poroussubstrate includes a member selected from the group consisting of: asemiconductor, a metal, carbon, graphene, activated carbon, carbonnanotubes, carbon nanofibers, anodized aluminum, or combinationsthereof.
 22. The electrode of claim 17, wherein the porous substrate isporous silicon.
 23. The electrode of claim 22, wherein the poroussilicon is doped.
 24. The electrode of claim 17, wherein the poroussubstrate is a particulate.
 25. The electrode of claim 24, wherein theparticulate is held together by the conductive coating.
 26. Theelectrode of claim 17, wherein the conductive coating includes amaterial selected from the group consisting of: metals, metal nitrides,metal silicides, metal carbides, metal borides, carbon or combinationsthereof.
 27. The electrode of claim 17, wherein the conductive coatingincludes a material selected from the group consisting of: titaniumnitride, tungsten nitride, tantalum nitride, or combinations thereof.28. An ultracapacitor, comprising: at least two current collectors eachcoupled to an electrode as recited in claim 17, wherein the currentcollectors are positioned such that the electrodes face one another; andan electrolyte disposed between the electrodes such that the electrolytecontacts at least a portion of the first side and at least a portion ofthe second side of the conductive coatings of each electrode.
 29. Theultracapacitor of claim 28, wherein the electrolyte contacts at leastabout 5% of the second side of the conductive coatings of eachelectrode.
 30. A charge storage system, comprising: a stack ofultracapacitor cells, comprising at least one ultracapacitor as recitedin claim 28.